Okay, it looks like the Space Mimic is practically finished apart filling out its description/flavour text and tactics.
How about this:
Space mimics are a highly intelligent spellcasting relative of the common mimic, usually encountered floating the depth of wildspace or hiding aboard spelljamming vessels. Unlike common mimics, space mimics have a culture of sorts based on the study of arcane lore. They are keen readers, preferring tomes on magic and astronomy. On the rare occasions a space mimic meets another of its kind they will debate arcane lore and exchange books or scrolls they've already read for new ones. Space mimics need spell books to cast their spells just like a wizard.
These monsters are just as amoral as common mimics, seeing most creatures as potential meals and sources of treasure. They will negotiate with powerful creatures, exchanging their services for food or arcane volumes, but weaker beings are there to be eaten.
A space mimic can have almost any dimensions, but usually is not more than 10 feet long. A typical space mimic has a volume of 150 cubic feet (5 feet by 5 feet by 6 feet) and weighs about 4,500 pounds.
Space mimics can speak and read common and at least two other languages, usually Draconic and Elven.
Combat
A space mimic disguises itself, often as a large chest or massive desk, and waits for victims to arrive. It then observes its potential prey and tries to lure a single victim away with an enchantment or illusion spell such as charm person or ghost sound, hoping to devour them without anyone noticing.
Space Mimics fight with spells as well as adhesive pseudopods as powerful as a common mimic's. They are crafty creatures who know how to use their spells to their best advantage - charming opponents and turning them on their companions, casting web to stymie their enemies' movements and so forth. If a battle turns against them a space mimic seeks to negotiate or fly away, but they fight to the death if cornered.